郯庐断裂带江苏段安丘-莒县断裂全新世活动及其构造意义

郯庐断裂带是中国东部重要的活动断裂带和边界构造带,其鲁苏段全新世活动断层的空间展布和古地震序列是地学关注的焦点问题,也是准确评价区域地震危险性的重要参数.以往研究工作多集中在郯庐断裂带地表地貌现象明显且有强震记录的山东段,而江苏段则研究程度相对较低,有关郯庐断裂带江苏段全新世活动断层范围和古地震序列问题存在争议.本文利用野外地质地貌调查、浅层地震勘探、钻孔联合剖面以及古地震探槽等多层次综合方法,重点开展郯庐断裂带江苏段全新世活动断层的分布和古地震序列研究.结果显示全新世时期,安丘-莒县断裂是郯庐断裂带江苏段的主要活动断层,且江苏全段该断层都是全新世活动断层.通过对比宿迁闸-皂河镇断裂南北安丘-莒县断裂的断层地貌和断层最新活动时间,并结合宿迁闸-皂河镇断裂在第四纪没有活动过等证据,推测该断层在全新世时期并不是区域阻碍破裂的断层.探槽揭示郯庐断裂带江苏段全新世两次古地震事件,事件Ⅰ限定在（6.2±0.3）-（13.4±0.7）ka B.P.之间,而事件Ⅱ限定在（2.5±0.1）ka B.P.到现今,全新世两次古地震间隔较长.基于构造类比法,安丘-莒县断裂具有深部孕震的构造特点,是区域未来强震的潜在发震构造.     

Rare destructive earthquakes in Europe: The 1904 Bulgaria event case

Seismic hazard is difficult to assess in regions of low strain rates. A major limitation often relates to the absence of large instrumentally recorded events precluding any comparison between seismological data and paleoseismic or morphotectonic informations. We take advantage of the 1904 M_s∼7.1 earthquake that struck the southern edge of stable Eurasia and investigate if morphotectonic and paleoseismic observations can provide a reliable estimate of the seismic potential of slow-slipping faults. We have conducted a paleoseismic study of the Krupnik normal fault thought to be responsible for the event. A section of the fault bearing remnants of a 2 m-high scarp has been selected at the base of triangular facets. The trenching site locates where the scarp cuts across colluviums washed from the bedrock facetted slopes. We excavated two neighbouring trenches, one across a well-preserved portion of the scarp, and one across a portion degraded by a landslide. The excavations reveal a set of coarse colluvial units faulted against bedrock and affected by secondary fissures. Faulting appears to have resulted from a single event with normal throw greater than 1.3 m that occurred before the emplacement of the landslide. Accelerator Mass Spectrometry (AMS) radiocarbon dates of charcoal samples are consistent with the interpretation that the Krupnik Fault slipped recently, most probably in 1904, after a long lasting (> 10 ka) period of quiescence. The morphotectonic and paleoseismic observations yield seismic moment estimates compatible with the instrumental magnitude of the event and indicate that destructive and infrequent earthquakes typify the regional seismic behaviour.     

Assimilation of Paleoseismic Data for Earthquake Simulation

— Simulation of the complete earthquake generation process requires assimilation of observational data on long-term behavior of faults over multiple rupture cycles. Paleoseismology, the study of pre-instrumental earthquakes from geological field investigations, provides the only observations of long-term fault behavior. Paleoseismic data present challenges for assimilation into models because data sets are small, sparse, analog, and contain qualitative uncertainties. Observations can be categorized as primary measurements from field data, or non-primary extrapolations or interpretations of primary data. Assimilation of non-primary data could yield biased simulation results. We present a primary data set for the San Andreas and Imperial faults for comparison with non-primary data in existing databases, and propose standard formats and structure for paleoseismic data assimilation into numerical simulation models .     

Logic-tree Approach for Probabilistic Tsunami Hazard Analysis and its Applications to the Japanese Coasts

For Probabilistic Tsunami Hazard Analysis (PTHA), we propose a logic-tree approach to construct tsunami hazard curves (relationship between tsunami height and probability of exceedance) and present some examples for Japan for the purpose of quantitative assessments of tsunami risk for important coastal facilities. A hazard curve is obtained by integration over the aleatory uncertainties, and numerous hazard curves are obtained for different branches of logic-tree representing epistemic uncertainty. A PTHA consists of a tsunami source model and coastal tsunami height estimation. We developed the logic-tree models for local tsunami sources around Japan and for distant tsunami sources along the South American subduction zones. Logic-trees were made for tsunami source zones, size and frequency of tsunamigenic earthquakes, fault models, and standard error of estimated tsunami heights. Numerical simulation rather than empirical relation was used for estimating the median tsunami heights. Weights of discrete branches that represent alternative hypotheses and interpretations were determined by the questionnaire survey for tsunami and earthquake experts, whereas those representing the error of estimated value were determined on the basis of historical data. Examples of tsunami hazard curves were illustrated for the coastal sites, and uncertainty in the tsunami hazard was displayed by 5-, 16-, 50-, 84- and 95-percentile and mean hazard curves.     

Active tectonics of the area of the two Kaliningrad earthquakes on September 21, 2004

Resulting from the seismotectonic study of the Sambian Peninsula based on the interpretation of remote sensing data (satellite images and digital elevation maps), lineaments have been identified. They may be interpreted as active faults and flexure-fracture zones. These active faults, which are expressed in the form of gentle linear swells or steps in the relief, have been found and studied during the carried out field works. There are many discovered paleoseismic dislocations in the studied areas of active faults: fracture displacements, marks of liquefaction (sand dykes), near-fault folds. These seismic dislocations may indicate seismic shocks of 7–8 intensity points occurring in the neighborhood of a modern Kaliningrad city in recent geological history. The identified active structures (Yantarny Fault Zone and Bakalino flexure-fracture zone) could control sources of the 2004 Kaliningrad earthquakes.     

The use of liquefaction features in paleoseismology: Lessons learned in the New Madrid seismic zone, central United States

A recent study in the New Madrid seismic zone demonstrates that large uncertainties, often involved but rarely expressed, in paleoliquefaction studies can be reduced by conducting detailed investigations at the most promising sites for dating liquefaction features. During the site investigations, care must be taken to collect samples that will provide close maximum and minimum dates for liquefaction features. It is advisable to use two-sigma calibrated dates, rather than one-sigma calibrated dates or radiocarbon ages, when estimating ages of liquefaction features. Well-constrained ages of individual liquefaction features should provide the basis for estimating the timing of paleoearthquakes and correlating features across a region. As uncertainty in ages of liquefaction features decreases, confidence in estimates of timing, source areas and magnitudes of paleoearthquakes increases. The New Madrid study also shows that modern or historic earthquakes that induced liquefaction in the same region and whose locations and magnitudes are fairly well know can serve as calibration events for paleoearthquakes. Future efforts that could further improve the usefulness of liquefaction feature in paleoseismology include (1) the development of new techniques for dating liquefaction features directly, (2) case studies of modern earthquakes that focus on the site and spatial distributions of liquefaction feature as well as geotechnical properties of liquefaction sites and (3) more rigorous quantification of uncertainties associated with estimates of timing, source areas and magnitudes of paleoearthquakes.     

LATE QUATERNARY ACTIVITY AND PALEOSEISMIC RUPTURE BEHAVIOR FOR THE SOUTHEAST SECTION OF THE GANZI-YUSHU FAULT

The Ganzi-Yushu Fault, the boundary of Bayan Har active tectonic block, Qiantang active tectonic block and Sichuan-Yunan active tectonic block, is a sinistral strike-slip fault zone with intensive Holocene activity. Thus, the study of activity characteristics and rupture behavior of paleoearthquakes in the late Quaternary on the Ganzi-Yushu Fault is of fundamental importance for understanding the future seismic risk of this fault. The southeast section of Ganzi-Yushu Fault is made up of three segments of Ganzi, Manigange and Dengke, where a M_S7.3 earthquake in 1866, a M_S7.7 earthquake in 1854 and a M_S7.3 in 1896 occurred, respectively. There is still lack of in-depth study on the active features and the cascading rupture possibility of these segments, which hindered the evaluation of seismic risk for the southeast section of Ganzi-Yushu Fault. By the means of field geological survey and micro topography measurement, this paper studied the geological and geomorphological features of the southeast section of the Ganzi-Yushu Fault. The results show that the Ganzi and Dengke segments show obvious extension movement, in addition to the left-lateral movement. For Manigange segment, the characteristics of the movement are mainly left-lateral strike-slip and thrusting, and the maximum vertical displacement of the Holocene strata is greater than 2m. In part areas, the movement is normal faulting, which perhaps relates to the left stepping zone in the local stress environment. Therefore, combining the research results such as the fracture distribution in different motion characteristics, rupture behavior of paleoearthquakes, and the distribution of historical earthquake surface ruptures, we divide the southeast section of Ganzi Yushu Fault into Ganzi, Manigange and Dengke segment, and consider the Yakou and the Dengke Basin as the stepovers and the segments' boundaries. As the small scale of impermanent barriers including Dengke Basin and the ridge near Yakou, of which the width is about 1~2km, they may be broken through in great earthquake rupture in future. A trench was excavated in Zhuqing township to investigate the paleoearthquakes on the Manigange segment, radiocarbon dating was employed and 3 paleoseismic events were revealed in the Zhuqing trench, which are the seismic events occurring respectively at 3875~3455BC, after 775BC, and the latest one that ruptured the surface. Compared with the previous results of paleoseismology in the southeast section of Ganzi-Yushu Fault, it is found that the paleoseismic events in the Manigange segment are obviously different with that in Ganzi segment and Dengke segment. Due to the lack of sufficient data on the southeast section of the Ganzi-Yushu Fault, it still needs further discussion whether the cascade-rupturing between these segments exists.     

RUPTURE CHARACTERISTICS OF LATE QUATERNARY STRONG EARTHQUAKES ON THE WESTERN BRANCH OF THE XIAOJIANG FAULT ZONE

The Xiaojiang fault zone is located in the southeastern margin of the Tibetan plateau, the boundary faults of Sichuan-Yunnan block and South China block. The largest historical earthquake in Yunnan Province, with magnitude 8 occurred on the western branch of the Xiaojiang Fault in Songming County, 1833. Research on the Late Quaternary surface deformation and strong earthquake rupture behavior on the Xiaojiang Fault is crucial to understand the future seismic risk of the fault zone and the Sichuan-Yunnan region, even crucial for the study of tectonic evolution of the southeastern margin of Tibetan plateau. We have some new understanding through several large trenches excavated on the western branch of the Xiaojiang fault zone. We excavated a large trench at Caohaizi and identified six paleoseismic events, named U through Z from the oldest to the youngest. Ages of these six events are constrained at 40000-36300BC, 35400-24800BC, 9500BC-500AD, 390-720AD, 1120-1620AD and 1750AD-present. The Ganhaizi trench revealed three paleoearthquakes, named GHZ-E1 to GHZ-E3 from the oldest to the youngest. Ages of the three events are constrained at 3300BC-400AD, 770-1120AD, 1460AD-present. The Dafendi trench revealed three paleoearthquakes, named E1 to E3 from the oldest to the youngest, and their ages are constrained at 22300-19600BC, 18820-18400BC, and 18250-present. Caohaizi and Ganhaizi trenches are excavated on the western branch of the Xiaojiang Fault, the distance between them is 400m. We constrained four late Holocene paleoearthquakes with progressive constraining method, which are respectively at 500-720AD, 770-1120AD, AD 1460-1620 and 1833AD, with an average recurrence interval of 370~440a. Large earthquake recurrence in the late Holocene is less than the recurrence interval of~900a as proposed in the previous studies. Thus, the seismic hazard on the Xiaojiang Fault should be reevaluated. We excavated a large trench at Dafendi, about 30km away south of Caohaizi trench. Combining with previous paleoseismological research, it is found that the western branch of Xiaojiang Fault was likely to be dominated by segmented rupturing in the period from late of Late Pleistocene to early and middle Holocene, while it was characterized by large earthquakes clustering and whole segment rupturing since late Holocene.     

西秦岭北缘断裂带特征地震平均复发间隔的确定和地震危险性评价

时间相依的地震危险性概率评估方法是最近10a来逐渐发展起来的，是一种将已获得的定量地质资料运用于活动断裂中—长期地震潜势概率评估的方法，从而使得在缺乏历史记载或仪器记录资料，但已获得断层平均滑动速率、同震位错、古地震年代序列等资料的活动断裂段上评估未来的发震概率成为可能。在定量计算活动断裂未来地震危险性的过程中，作为输入参数之一的特征地震平均复发间隔是一个至关重要的参数，它的确定将直接影响到概率计算的结果。对研究断裂上已获得的历史地震资料(H)、地质资料(G)和古地震资料(P)，笔采用了时间可预报(T)和准周期(Q)两种模式分别计算其平均复发间隔，比单一的只假定一种复发模式计算更具有完善性和可靠性。在叙述该方法的同时，以西秦岭北缘断裂为例，详细阐述了该断裂上特征地震平均复发间隔的确定，并在此基础上对西秦岭北缘断裂未来地震潜势作了定量评估。     

The Pitman Canyon paleoseismic record: A re-evaluation of southern San Andreas Fault segmentation

Correlation of three well-resolved paleoseismic records, including the Pitman Canyon site with its emerging record, presents a new possibility to understand fault segmentation. To be a useful concept, fault segment boundaries must be relatively stationary over multiple seismic cycles and must appear frequently in limiting the rupture extent of earthquakes; thus, sites on the same segment should share more paleoseismic events than those on different segments. A conclusive event correlation between sites is difficult or impossible due to dating uncertainties. However, often the data are adequate to preclude correlation and thus provide firm limits on rupture extent for those events. Thus clear non-correlations provide more information about segmentation than do unprovable potential correlations.The southern end of the most recent rupture in 1857, between Wrightwood and Pitman Canyon, is often used to define a segment boundary. However, there is an absence of significant non-correlation between the previous five Pitman Canyon events and the Wrightwood events. While both Pallett Creek and Wrightwood ruptured in 1857, only two of the previous five Wrightwood events can correlate with Pallett Creek events, which may or may not indicate that they actually do. These paleoseismic records do not support the existence of a segment boundary between Wrightwood and Pitman Canyon as defined by the 1857 rupture extent, suggesting a reevaluation of southern San Andreas Fault segmentation, and using historic ruptures to define segments in general.     

则木河断裂带大箐梁子附近古地震组合探槽研究

古地震事件定年具不确定性,为了增加地震事件定年的可信度,文中通过断层活动方式、沉积环境和地震微地貌的综合分析来研究则木河断裂带上的古地震事件。在大箐梁子附近开挖组合探槽,揭露了3次古地震事件,分别距今约160a、3100a和5500～8900a,复发间隔为3000a左右;讨论了与走滑断层型地震相关的沉积过程,揭示了走滑断层在山坡部位形成鼓包和反向陡坎情况下的沉积模式     

STUDY ON PALEOEARTHQUAKES ALONG THE JINGHE SECTION OF BOLOKENU-AQIKEKUDUKE FAULT

The Bolokenu-Aqikekuduk fault zone(B-A Fault)is a 1 000km long right-lateral strike-slip active fault in the Tianshan Mountains. Its late Quaternary activity characteristics are helpful to understand the role of active strike-slip faults in regional compressional strain distribution and orogenic processes in the continental compression environment, as well as seismic hazard assessment. In this paper, research on the paleoearthquakes is carried out by remote sensing image interpretation, field investigation, trench excavation and Quaternary dating in the Jinghe section of B-A Fault. In this paper, two trenches were excavated on in the pluvial fans of Fan2b in the bulge and Fan3a in the fault scarp. The markers such as different strata, cracks and colluvial wedges in the trenches are identified and the age of sedimentation is determined by means of OSL dating for different strata. Four most recent paleoearthquakes on the B-A Fault are revealed in trench TC1 and three most recent paleoearthquakes are revealed in trench TC2. Only the latest event was constrained by the OSL age among the three events revealed in the trench TC2. Therefore, when establishing the recurrence of the paleoearthquakes, we mainly rely on the paleoearthquake events in trench TC1, which are labeled E1-E4 from oldest to youngest, and their dates are constrained to the following time ranges: E1(19.4±2.5)~(19.0±2.5)ka BP, E2(18.6±1.4)~(17.3±1.4)ka BP, E3(12.2±1.2)~(6.6±0.8)ka BP, and E4 6.9~6.2ka BP, respectively. The earthquake recurrence intervals are(1.2±0.5)ka, (8.7±3.0)ka and(2.8±3)ka, respectively. According to the sedimentation rate of the stratum, it can be judged that there is a sedimentary discontinuity between the paleoearthquakes E2 and E3, and the paleoearthquake events between E2 and E3 may not be recorded by the stratum. Ignoring the sedimentary discontinuous strata and the earthquakes occurring during the sedimentary discontinuity, the earthquake recurrence interval of the Jinghe section of B-A Fault is ~1~3ka. This is consistent with the earthquake recurrence interval(~2ka)calculated from the slip rate and the minimum displacement. The elapsed time of the latest paleoearthquake recorded in the trench is ~6.9~6.2ka BP. The magnitude of the latest event defined by the single event displacement on the fault is ~M_W7.4, and a longer earthquake elapsed time indicates the higher seismic risk of the B-A Fault.     

Using borehole core analysis to reveal Late Quaternary paleoearthquakes along the Nankou-Sunhe Fault,Beijing

The Nankou-Sunhe Fault is a buried active normal fault that traverses the urban area of Beijing.Its seismic risks have caused considerable concerns.This paper studies paleoearthquakes along this fault by analyzing and correlating bore-hole cores obtained from triple-tube coring,incorporating experience acquired from trenching.As a result,a model for identifying earthquake-derived colluvium by sediment-core analysis is proposed.Triple-tube coring technique is useful to collect continuous undis-turbed soil core near the Nankou-Sunhe Fault.By identifying fault-scarp colluviums,determining cumulative displacement,and analysing stratum thickening on the hanging wall,we are able to establish a preliminary paleoearthquake sequence consisting of 13 surface-rupturing events since 60 ka.The seismic history can be divided into three periods based on different recurrence intervals.Between 60 and 40 ka,three earthquakes occurred with recurrence interval of ～10 ka.From 40 to 25 ka,there were six earthquakes with the recurrence interval of about 2.5 ka.In the last 25 ka,four earthquakes have taken place with the recurrence interval varying considerably.The recurrence interval between the last three events is ～5 ka.Smaller recurrence intervals correspond to stages of faster fault slip.The coseismic displacement of a single event is 0.8 to 2.2 m,average 1.4 m,largely equivalent to moment magnitudes 6.7-7.1.This study demonstrates the feasibility of bore-hole drilling in investigating paleoearthquakes along normal faults.It also suggests that closely spaced boreholes with continuous undisturbed cores are essential for reconstructing the complete paleoearthquake sequence.     

Long-term seismicity in regions of present day low seismic activity: the example of western Europe

In western Europe, the knowledge of long-term seismicity is based on reliable historical seismicity and covers a time period of less than 700 years. Despite the fact that the seismic activity is considered as low in the region extending from the Lower Rhine Embayment to England, historical information collected recently suggests the occurrence of three earthquakes with magnitude around 6.0 or greater. These events are a source of information for the engineer or the scientist involved in mitigation against large earthquakes. We provide information relevant to this aspect for the Belgian earthquake of September 18, 1692. The severity of the damage described in original sources indicates that its epicentral intensity could be IX (EMS-98 scale) and that the area with intensity VII and greater than VII has at least a mean radius of 45 km. Following relationships between average macroseismic radii and magnitude for earthquakes in stable continental regions, its magnitude M_s is estimated as between 6.0 and 6.5. To extend in time our knowledge of the seismic activity, we conducted paleoseismic investigations in the Roer Graben to address the question of the possible occurrence of large earthquakes with coseismic surface ruptures. Our study along the Feldbiss fault (the western border of the graben) demonstrates its recent activity and provides numerous lines of evidence of Holocene and Late Pleistocene large earthquakes. It suggests that along the 10 km long Bree fault scarp, the return period for earthquakes with magnitude from 6.2 to 6.7 ranges from 10,000 to 20,000 years during the last 50,000 years. Considering as possible the occurrence of similar earthquakes along all the Quaternary faults in the Lower Rhine Embayment, a large earthquake could occur there each 500–1000 years. These results are important in two ways. (i) The evidence that large earthquakes occur in western Europe in the very recent past which is not only attested by historical sources, but also suggested by paleoseismic investigations in the Roer Graben. (ii) The existence of a scientific basis to better evaluate the long-term seismicity in this part of Europe (maximal magnitude and return period) in the framework of seismic hazard assessment.     

Major earthquakes of the southern Gornyi Altai in the Holocene

The paper describes geological and geomorphological features of ancient major earthquakes that occurred in the Holocene in the zone of the Altai earthquake of September 27, 2003 (southern Gornyi Altai). Fossil earthquakes occurred in the regions of the Northern and Southern Chuya ridges and Chuya and Kurai basins; their sources reached the surface and formed systems of coseismic ruptures oriented SE-NW or E-W. Secondary (gravitational and vibrational) paleoseismic deformations were represented by rockfalls, landslides, and gryphons in near-field zones of these paleoevents. They were spread in an area of about 2000–2500 km² coinciding, on the whole, with the position of source zones. Paleoseismogeological investigations showed that, apart from earthquakes of the 20th and 21st centuries, eight seismic events with magnitudes M = 7.0–8.0 occurred in the region over the last 8500 years. Ancient strong earthquakes that produced the observed paleoseismic ruptures occurred approximately 230–300, 1000, 1700, 2300, 3500, 4500, 5200, and 8500 years ago. Therefore, the recurrence interval averaged 1400 yr for earthquakes with a magnitude of about 7.0 and 2100 yr for events with a magnitude of about 7.5.     

LATE QUATERNARY PALEOEARTHQUAKES ON THE MIDDLE SEGMENT OF THE LIJIANG-XIAOJINHE FAULT,SOUTHEASTERN TIBET

More attention has been paid to the late Quaternary activity of the boundary fault of the Sichuan-Yunnan block in eastern Tibet. The Lijiang-Xiaojinhe Fault (LXF) locates along the boundary of the northwest Sichuan and central Yunnan sub-blocks in the Sichuan-Yunnan block. Clear displaced landforms show that the fault has undergone strong late-Quaternary activity. However there is no surface-rupturing earthquake occurring on the LXF in the historical record. The LXF crosses the city of Lijiang, one of the most important tourist cities in Southwest China. The rupture behavior on this fault remains unclear and it is hard to assess its seismic hazard in the future. In this study, on the base of the interpretation of high-resolution satellite imagery, we chose the middle segment of the LXF and dug three trenches at Muzhuda, Hongxing, and Gantangzi sites to constrain the ages of paleoearthquakes combined with radiocarbon dating and OxCal modeling. The Muzhuda trench shows that at least three events occurred on the middle segment of the LXF at 7 940~6 540a BP, 4 740~4 050a BP and 1 830~420a BP, respectively. The Hongxing trench indicates that the LXF underwent two events at 5 120~3 200a BP and 2 100~1 220a BP. The Gantangzi trench reveals at least three paleoearthquakes at 44 980~17 660a BP, 7 210~3 810a BP and 2 540~1 540a BP, respectively. The events in the Gantangzi trench might be incomplete because of stratigraphic gap. These three trenches indicate that three events occurred on the middle segment of the LXF in the Holocene at 7 940~7 210a BP, 4 740~4 050a BP and 1 830~1 540a BP, respectively. Large earthquakes on the middle segment of the LXF appear to fit the quasi-periodic model with the mean recurrence interval of~3 000a and the estimated magnitude 7.5. Given the strong late-Quaternary activity of the middle segment of the LXF and a long elapsed time, we propose that the middle segment of the LXF might have a high seismic hazard potential in the near future.     

THE SLIP RATE AND PALEOEARTHQUAKES OF THE YUMEN FAULT IN THE NORTHERN QILIAN MOUNTAINS SINCE THE LATE PLEISTOCENE

The Yumen Fault lies on the west segment of the north Qilian Fault belt and adjacent to the Altyn-Tagh Fault,in the north margin of the Tibet Plateau.The tectonic location of the Yumen fault is special,and the fault is the evidence of recent activity of the northward growth of Tibetan plateau.In recent twenty years,many researches show the activity of the Yumen Fault became stronger from the early Pleistocene to the Holocene.Because the Yumen Fault is a new active fault and fold belt in the Qilian orogenic belt in the north margin of the Tibet Plateau,it is important to ascertain its slip rate and the recurrence interval of paleoearthquakes since the Late Pleistocene.Using the satellite image interpretation of the Beida river terrace,the GPS measurement of alluvial fans in front of the Yumen Fault and the trench excavation on the fault scarps,two conclusions are obtained in this paper.(1) The vertical slip rate of the Yumen Fault is about 0.41~0.48mm/a in the Holocene and about 0.24~0.30mm/a in the last stage of the late Pleistocene.(2) Since the Holocene epoch,four paleoearthquakes,which happened respectively in 6.12~10.53ka,3.6~5.38ka,1.64~1.93ka and 0.63~1.64ka,ruptured the surface scarps of the Yumen Fault.Overall,the recurrence interval of the paleoseismic events shortens gradually and the activity of the Yumen Fault becomes stronger since the Holocene.Anther characteristic is that every paleoearthquake probably ruptured multiple fault scarps at the same time.     

Time-dependent probabilistic seismic hazard assessment for Taiyuan,Shanxi Province,China, and the surrounding area

Taiyuan is a city in Shanxi Province, China, and possesses serious seismic hazard. In this study, we constructed a time-dependent seismic hazard model for Taiyuan and the surrounding area based on several major-earthquake seismogenic structures for which historical and paleoseismic event data were available. With the time-dependent model, we calculated the distribution of peak ground acceleration with 10% probability of exceedance in the next 50 years in Taiyuan and the surrounding area, and compared the results with those calculated using the time-independent model. The results showed that the entire area around Taiyuan has a higher seismic hazard with the time-dependent model than that with the time-independent model. The Jiaocheng and Hengshan faults have much higher seismic hazard. Applying the model only to Taiyuan showed that the city has higher seismic hazard with the time-dependent model than that with the time-independent model. In particular, in the western part of Taiyuan, the seismic hazard is quite high because of the proximity of the area to the Jiaocheng Fault.     

THE HOLOCENE SEISMIC EVIDENCE ON SOUTHERN SEGMENT OF THE RED RIVER FAULT ZONE

Nine earthquakes with M≥6 have stricken the northern segment of the Red River fault zone since the historical records, including the 1652 Midu M7 earthquake and the 1925 Dali M7 earthquake. However, there have been no earthquake records of M≥6 on the middle and southern segments of the Red River Fault, since 886 AD. Is the Red River fault zone, as a boundary fault, a fault zone where there will be not big earthquake in the future or a seismogenic structure for large earthquake with long recurrence intervals?This problem puzzles the geologists for a long time. Through indoor careful interpretation of high resolution remote sensing images, and in combination with detailed field geological and geomorphic survey, we found a series of fault troughs along the section of Gasha-Yaojie on the southern segment of the Red River fault zone, the length of the Gasha-Yaojie section is over ten kilometers. At the same time, paleoseismic information and radiocarbon dating result analysis on the multiple trenches show that there exists geological evidence of seismic activity during the Holocene in the southern segment of the Red River fault zone.     

On the relevancy of the combination of macroseismic and paleoseismic data

Earthquakes that occurred in the Baikal seismic region in 1725, 1742, 1769, and 1829 are studied on the basis of original macroseismic information. Due to the fact that their parameters were previously determined using the combination of macroseismic and paleoseismic data, the goal of our investigation is to verify how well the solutions agree with the macroseismic evidence. Careful examination of macroseismic data includes, first of all, the searching for original sources, which cannot be replaced by any other data types, for instance, paleoseismic information characterized by questionable reliability. The completeness of analysis is achieved when different components are inspected separately before mixing. In the case when unequivocal data interpretation is impossible, it is better to consider different alternative solutions characterized by relatively narrow error ranges. Variants can be weighted correspondingly (at least, evaluated qualitatively). Otherwise we have to deal with the so-called “average” solutions, often useless due to great determination errors. Magnitudes of all earthquakes estimated previously on the basis of macroseismic and paleoseismic data are not confirmed by the original macroseismic information, and revised magnitudes are essentially lower.